King Kai (界王Kaiô, literally, King of the Worlds) or North Kai (北の界王Kita no Kaiô, literally, King of the Worlds of the North) is a deity and Goku's final martial arts teacher, and later mentor. He also taught Yamcha, Tien Shinhan, Piccolo and Chiaotzu. His planet is at the end of a long winding road known as Snake Way, where he teaches powerful, new techniques such as the Spirit Bomb and Kaio-ken. Despite the fact that his planet is around 100 feet in diameter, its gravity is ten times greater than Earth's. He has a monkey named Bubbles, a motor-mouth grasshopper named Gregory, and owns a 50's-style red Cheverolet Sedan (a nod to the fact that Akira was born in the 50's). He has a "strong sense of humor" and tells very bad jokes, although he possesses great wisdom and knowledge about the universe. He specializes in universal telepathic links. His best fighter, aside from Goku, is Olibu.

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Goku arrives on King Kai's Planet after months of running to train for the coming battle against the Saiyans. King Kai requires that Goku prove himself by telling a joke. Then he learns Goku is not here to train in comedy. Before the training began, King Kai warned Goku that Saiyans approaching Earth were stronger then he was, so Goku would need to surpass him if he wanted to stop them.

As part of Goku's training, he tells him to chase his pet monkey Bubbles, who happens to be very fast and has adjusted to the planet's gravity which is simular to the gravity on planet Vegeta, about 10 times the gravity of earth. After three weeks, Goku caught Bubbles. He then instructed him to hit a cricket, Gregory, on the head with a heavy mallet. In time Goku accomplished this as well. Goku soon found that chasing Bubbles increased his speed greatly, and attempting to hit Gregory increased his strength massively, though it should be noted that the second task of hitting Gregory did not appear in the original manga (Gregory did not even exist in the original manga, with King Kai immediately training Goku directly after catching Bubbles), and was only in the anime. Soon before the Saiyans' arrival on Earth, Goku left for Earth after being revived, with King Kai witnessing the battle. Although Goku completed his physical training King Kai sensed he still needed to work on his humor.

Soon after Vegeta retreated, Yamcha, Tien, Piccolo and Chiaotzu took the journey on Snake Way and arrived on King Kai's Planet faster than Goku for special training. King Kai warned Goku never to encounter Frieza, an intergalactic tyrant and (at the time) the most powerful being in the universe. Yamcha, Tien and Chiaotzu receive training from King Kai, with Piccolo opting out doing most of the training by himself (believing it to be silly and pointless catching Bubbles in mere seconds, compared to Goku's forty days, since the greater gravity is similar to his techinque of wearing weighted clothing, which he frequently does in his training). However, he was surprised to see improvements in the other three's skills during a sparring match, though this took place only in the anime. As King Kai witnessed the battle against Frieza, the recently-deceased Ginyu Force excluding Captain Ginyu, fought the Z Fighters in training (without Piccolo who had been revived and sent to Namek) and were defeated by the Z Fighters and sent to Hell, which was rather impressive considering King Kai considered them to be most fearsome group in the universe, and at least five times stronger than Goku prior to training. This however happened in the anime only, and when considering that it took Goku over 150 days to improve to a point where he could defeat Nappa, the weaker of the two Saiyans that were considered to be nothing in comparison to even one member of the Ginyu Squad, it seems unlikely that they could have achieved such a dramatic increase in power in such a short time. However it may be worth noting that they were able to spar with each other in addidtion to King Kai's training where as Goku was trained alone.

He soon noticed that Frieza was not defeated as originally thought, and Frieza injured Piccolo by piercing his chest, then killed Krillin. This heinous action caused Goku to transform into a Super Saiyan. Eventually Goku, in his new form, overwhelmed Frieza, leaving him to attempt to destroy Namek. Although not instantly destroyed (due to Frieza's fear of being caught in its explosion), Frieza estimated that the planet would survive only five minutes longer. With Kami now revived, and the Dragon Balls collected, King Kai devised a plan to revive all those who had perished at the hands of Frieza and his minions throughout the past year. This would in turn revive Guru, who died because of the pain of seeing his children die by Frieza's great cruelty. Frieza powered up to full power (half of which he was using prior to Goku's transformation) and began to fight Goku. As Shenron resurrected all those who were killed by Frieza, Guru was brought back and Porunga returned in front of the newly revived Dende. King Kai made telepathic contact with Guru to have Dende ask Porunga to teleport everyone off Namek to Earth, except Frieza. Goku interrupted, and requested that he, too, be left behind in order to ensure Frieza's defeat. King Kai objected, insisting that Goku's transformation had clouded his judgment, but Goku's determination eventually prevailed. With their plan revised, Guru sent Dende to make the final wish to Porunga to transport everyone off Namek to Earth, except Goku and Frieza. After defeating Frieza, Goku flew to Frieza's ship but found it was too heavily damaged, and seemingly died with the planet. Yamcha informed Bulma through King Kai of Goku's death. However, Bulma pointed out that with the Namekian Dragon Balls, Goku could be revived, along with Krillin and Chiaotzu, who were both killed twice. King Kai then said that though Chiaotzu could be revived, Goku and Krillin had died on Namek, and as Porunga resurrects people to the location at which they died, they would die again in the vacuum of space if they were wished back. However, Vegeta suggested voting Krillin and Goku to the Earth's Check-In Station to the spirit world, then wish them back.

King Kai (as well as everyone else aware of the events transpiring) was shocked when a boy from the future (Future Trunks) arrived and effortlessly defeated Frieza and his father, King Cold, who had traveled to Earth to exact revenge for Frieza's defeat.

Kami's fusion with Piccolo had left the Earth without a guardian, and the Dragon Balls had become inert, so when Goku planned to travel to New Namek to draft a new guardian, he enlisted King Kai's help in locating the planet.

Later, when Cell had been bested by Gohan in battle, he initiated his self-destruction. Having only seconds to react, Goku used Instant Transmission to transport himself and the android to King Kai's planet. Cell's subsequent self-destruction killed Goku and King Kai (as well as King Kai's pets), who soon realized that Cell must have survived the explosion. With the help of King Kai's telepathy, Goku provided spiritual support for Gohan, who had been forced to engage Cell once again. Afterwards, Goku tells everyone after declining revival that King Kai had also declined to be revived so he could guide Goku around Other World.